1. Field of the Invention
The present application relates to the deposition of a layer of material onto carbonaceous substrates, and, more particularly, to the deposition of titanium nitride onto cast iron.
2. Description of Related Art
Titanium nitride coatings on substrates are used to provide improved wear properties to the substrate materials. For example, cast iron foundry tools are exposed to sand abrasion, and it is desired to resist the wear resulting from such sand abrasion. Titanium nitride overcoatings could provide such wear resistance.
The deposition of titanium nitride onto substrates is disclosed in, for example, U.S. Pat. No. 4,197,175, using a reactive evaporation (RE) process. The deposition method comprises evaporating titanium in a vacuum evaporator. The evaporated titanium atoms deposit onto a substrate that is simultaneously bombarded with argon ions to grow a hard coating of titanium nitride using nitrogen gas that is introduced into the process chamber.
Prior to depositing titanium nitride onto the substrate surface, the substrate is bombarded using an argon-ion plasma. The Ar-ion bombardment accomplishes the task of sputter-cleaning the substrate surface. This physical sputtering cleans the substrate surface of foreign atoms and atom-species such as oxides that may be present, as well as surface hydrocarbons. The sputtering also removes a small amount of the substrate material itself, but this can be controlled to be minimal.
Chemically-reactive gases or chemically-reactive plasmas are not used to clean the surface of oxides and other surface contaminants. Instead, surface cleaning is accomplished by physical sputtering. This is disadvantageous for substrate materials which are sputter-sensitive, such as with carbonaceous materials like cast iron.
Once the surface is sputter-cleaned, heating of the substrate is accomplished by bombarding the substrate with electrons. Titanium nitride is then grown at the desired temperature with simultaneous argon ion bombardment of the film.
With the RE process, it has been observed that the deposition of titanium nitride on the surfaces of high carbon-containing materials, such as cast iron, does not provide good wear properties. The principal reason for this is the graphite that is in cast iron.
Most types of cast iron, such as the commonly-used gray cast iron, contain free graphite, which is added to aid in machinability and improved wear properties due to its lubricating ability. Without subscribing to any particular theory, it appears that the physical sputtercleaning step used in the above-described RE process results in the preferential sputtering of cast iron relative to the free graphite, due to the much lower sputter yield of graphite compared to that of iron. The result of this is that graphite is left on the surface of the cast iron prior to the deposition of the titanium nitride coating. Since graphite itself is brittle, the titanium nitride coating that is deposited onto the cast iron surface has poor adhesion and poor wear properties.
The use of neutral hydrogen is known to remove carbon from the surface of carbon-containing steels at temperatures higher than 700.degree. C. through the following reaction: EQU C+4H.fwdarw.CH.sub.4.
However, this technique is ill-suited for preparing a surface prior to the deposition of titanium nitride. At a temperature of 700.degree. C., carbon atoms in the cast iron are mobile. Once the carbon atoms on the cast iron surface are removed by hydrogen, the carbon atoms underneath the surface can diffuse to the surface, driven by the carbon concentration gradient, and then be removed from the surface by the neutral hydrogen gas. This high temperature process generates a decarburized and soft surface layer. This is described in the Metals Handbook, 9th Edition, Vol. 1, "Properties and Selection", American Society for Metals, 1978, pp. 66,301, 673-674, and 705. This soft substrate is ill-suited for supporting a hard coating of titanium nitride and would result in poor wear properties of the coating. Furthermore, this cleaning procedure is not applicable for temperature-sensitive substrates, since high temperature (&gt;700.degree. C.) is required.
The use of hydrogen plasma is well-known to remove surface contaminants, such as oxides and hydrocarbons, from substrates at room temperature. This is described by M. J. Vasile et al, "Mass Spectroscopy of Plasmas", in Plasma Diagnostics, Vol. I, Discharge Parameters and Chemistry, Edited by O. Auciello and D. Flamm, Academic Press, 1989.
However, these techniques do not teach the proper preparation of the surface for the subsequent deposition of hard films of titanium nitride that adhere to the surface nor substrate heating to the desired temperature for the film deposition.
Thus, a process is needed that provides improved adhesion and wear properties of titanium nitride deposited onto the surfaces of materials containing carbon in the form of graphite, such as found in certain commonly-used cast irons.